3 God created the heavens and the earth, but not only half—He created all the heavens and all the earth, creating the essence with the form. For He is not an inventor of figures, but the Creator even of the essence of beings. Further let them tell us how the efficient power of God could deal with the passive nature of matter, the latter furnishing the matter without form, the former possessing the science of the form without matter, both being in need of each other; the Creator in order to display His art, matter in order to cease to be without form and to receive a form. But let us stop here and return to our subject.
“The earth was invisible and unfinished.” In saying “In the beginning God created the heavens and the earth,” the sacred writer passed over many things in silence, water, air, fire and the results from them, which, all forming in reality the true complement of the world, were, without doubt, made at the same time as the universe. By this silence, history wishes to train the activity or our intelligence, giving it a weak point for starting, to impel it to the discovery of the truth. Thus, we are not told of the creation of water; but, as we are told that the earth was invisible, ask yourself what could have covered it, and prevented it from being seen? Fire could not conceal it. Fire brightens all about it, and spreads light rather than darkness around. No more was it air that enveloped the earth. Air by nature is of little density and transparent. It receives all kinds of visible object, and transmits them to the spectators. Only one supposition remains; that which floated on the surface of the earth was water— the fluid essence which had not yet been confined to its own place. Thus the earth was not only invisible; it was still incomplete. Even today excessive damp is a hindrance to the productiveness of the earth. The same cause at the same time prevents it from being seen, and from being complete, for the proper and natural adornment of the earth is its completion: grain waving in the valleys— meadows green with grass and rich with many coloured flowers— fertile glades and hill-tops shaded by forests. Of all this nothing was yet produced; the earth was in travail with it in virtue of the power that she had received from the Creator. But she was waiting for the appointed time and the divine order to bring forth.
Source: Nine Homilies of Hexaemeron (New Advent)